Sunan an-Nasa'i, Volume 2, Book 19, Hadith 1575 (DarusSalam) ⓘ
It was narrated that Simak said: "I asked Jabir: 'Did the Messenger of Allah deliver the Khutbah standing?' He said: 'The Messenger of Allah used to deliver the Khutbah standing, then he would sit for a while, then stand up again.'"
أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ سِمَاكٍ، قَالَ سَأَلْتُ جَابِرًا أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَخْطُبُ قَائِمًا قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَخْطُبُ قَائِمًا ثُمَّ يَقْعُدُ قَعْدَةً ثُمَّ يَقُومُ .
also referenced under: Hadith 1574 (Reference) & Book 19, Hadith 19 (In-Book)